Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity.
In one instance, a ribbon ion beam is used to implant the workpiece. A ribbon ion beam cross-section has a long dimension and a short dimension. The long dimension, for example, may be referred to as a width or x-direction, though other orientations are possible. The ribbon ion beam may be formed using a parallelizing lens or may be a scanned spot beam.
Occasionally, the ribbon ion beam may not be perfectly uniform. In the case where the ribbon beam is a scanned spot beam, non-uniformities in the ribbon are typically minimized by scanning the spot beam entirely or almost entirely off of the workpiece and by adjusting the time dependence of the scanning of the spot. However, under some circumstances, for instance when the spot beam current is large or the beam energy is small, it is common for the spot size of the ion beam to increase. The requirement of scanning the spot beam entirely or nearly entirely off the workpiece has the consequence that, for a large spot beam, relatively little beam current is utilized on the workpiece, which reduces the productivity of the implanter. The technique of minimizing non-uniformities by adjusting the time dependence of the scanning of the spot is known to be more complicated and less likely to succeed as the spot size increases.
Ribbon ion beam uniformity is one factor that affects implantation. Non-uniform ribbon ion beams may result in imprecise doping or implantation. For example, more heavily-doped stripes may be formed on the surface of a workpiece. An incorrect dose may cause deleterious yield effects if the devices are non-functioning due to the increased or decreased dose. Therefore, there is a need in the art for uniformity during implantation and, more particularly, uniformity during implantation on scanned spot beam implanters. It is with respect to these and other considerations that the present improvements have been needed.